Metal nanowire array and method for fabricating the same

ABSTRACT

A method for fabricating a metal nanowire array ( 30 ) includes the following steps: providing a carbon nanotube array ( 10 ) which includes a number of carbon nanotubes ( 14 ), immersing the carbon nanotube array in a colloidal solution ( 20 ) which contains a solvent ( 22 ) and a number of metal nanocrystals ( 24 ) dispersed in the solvent so as to self-assemble the metal nanocrystals on exterior surfaces of the carbon nanotubes; taking the carbon nanotube array out of the colloidal solution; and heating the metal nanocrystals on the surfaces of the carbon nanotubes in a manner such that the metal nanocrystals are fused into a metal nanowire array.

BACKGROUND

1. Technical Field

The present invention relates generally to nanomaterials, and more particularly to a metal nanowire array and a method for fabricating the same.

2. Discussion of Related Art

Miniaturization of integrated circuits is necessary for meeting the demand of processing data at higher speeds. Although some existing photolithography based technologies are useful in miniaturization, it is generally recognized that the existing technologies will reach their fundamental limits in the near future because of the wavelengths of the light sources used.

An alternative technology is the “bottom-up” assembly of integrated arrays of nanometer-scaled circuits from metal and semiconductor nanocrystals. In respect of this alternative technology, three key objectives must be accomplished. The first is the self-assembly of nanometer-scaled structures from nanocrystals dispersed in colloidal solution. The second is the self-organization of these structures on technologically relevant substrates. The third is the processing of these organized nanocrystal assemblies into robust structures suitable for practical applications.

In recent years, some exciting progress toward the first and second objects has been made. However, relatively little progress has been reported in the third area. Accordingly, a metal nanowire array with a robust structure suitable for practical applications and a method of fabricating such a metal nanowire array are desired.

SUMMARY

In one embodiment, a metal nanowire array includes an array of metal nanowires extending in a common direction, each metal nanowire including a core portion containing at least one carbon nanotube, and a cladding portion enclosing the core portion therein which includes metal polycrystalline.

In another embodiment, a method for fabricating a metal nanowire array includes the following steps: providing a carbon nanotube array which includes a number of carbon nanotubes; immersing the carbon nanotube array in a colloidal solution which contains a solvent and a number of metal nanocrystals dispersed in the solvent for self-assembling the metal nanocrystals on exterior surfaces of the carbon nanotubes; taking the carbon nanotube array out of the colloidal solution; and heating the metal nanocrystals on the surfaces of the carbon nanotubes in a manner such that the metal nanocrystals are fused into the metal nanowire array.

This and other features and advantages of the present invention as well as the preferred embodiments thereof and a metal nanowire array and techniques for fabricating metal nanowire array in accordance with the invention will become apparent from the following detailed description and the descriptions of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present metal nanowire array and related manufacture method can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present method. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views

FIG. 1 is a cross sectional schematic view of a metal nanowire array in accordance with the preferred embodiment;

FIG. 2 is a flow chart of a method for fabricating a metal nanowire array in accordance with the preferred embodiment; and

FIGS. 3A-3D are schematic views showing successive stages of the method for fabricating a metal nanowire array in accordance with the preferred embodiment.

DETAILED DESCRIPTION 0F PREFERRED EMBODIMENTS

Referring now to FIG. 1, a metal nanowire array 100 in accordance with the preferred embodiment includes a substrate 110 and an array of metal nanowires 120 extending in a substantially common direction. Each of the metal nanowires 120 includes a core portion 122 and a cladding portion 124. The core potion 122 is enclosed in the cladding portion 124. It should be noted that the cladding portion 124 could be partially or entirely formed on/enclosing an exterior surface of the core portion 122 and which should be considered to be within the scope of the present invention. The core portion 122 may include a single carbon nanotube or a plurality of bundled carbon nanotubes. The carbon nanotubes may be formed on the substrate 110. The cladding portion 124 includes metal polycrystalline, the metal can be selected from the group consisting of gold, silver, copper, tin, nickel, and germanium. Because of the strength of the aligned structure of the metal nanowire array 100, it is suitable for practical applications. The metal nanowire array 100 can be fabricated by the following method:

Referring now to FIG. 2, a method in accordance with the preferred embodiment includes the steps (in no particular order) of:

step 210: providing a carbon nanotube array 10 which includes a number of carbon nanotubes 14;

step 220: immersing the carbon nanotube array 10 in a colloidal solution 20 of metal nanocrystals 24 for self-assembling the metal nanocrystals 24 on exterior surfaces of the carbon nanotubes 14;

step 230: taking the carbon nanotube array 10 out of the colloidal solution 20; and

step 240: heating the metal nanocrystals assembled on exterior surfaces of the carbon nanotubes 14 to form a continuous polycrystalline metal nanowire array 40.

In step 210, referring to FIG. 3A, the carbon nanotube array 10 includes a substrate 12 and a number of carbon nanotubes 14 formed on the substrate 12. The carbon nanotubes 14 extend in a common direction, e.g. perpendicularly from the substrate. A method for preparing such a carbon nanotube array with well-aligned carbon nanotubes is disclosed, for example, in US patent 20040053053A1 by Jiang, KaiLi. et al, which is incorporated herein by reference. The method includes the following steps: providing a smooth substrate; depositing a metal catalyst layer on a surface of the substrate; heating the treated substrate to a predetermined temperature in flowing protective gas; and introducing a mixture of carbon source gas and protective gas for 5-30 minutes, thus forming a carbon nanotube array extending from the substrate. A patterned carbon nanotube array can be prepared by forming a patterned catalyst layer.

In step 220, referring to FIG. 3B, the carbon nanotube array 10 is immersed into a colloidal solution 20 of a metal nanocrystals for a period of time. The colloidal solution 20 contains a solvent 22 and a number of metal nanocrystals 24 dispersed in the solvent 22. The solvent 22 can be selected from the group consisting of water, chloroform, hexylene glycol, and alcohols containing a number of less than 5 carbon atoms (i.e. methanol, ethanol, propanol and butanol). The particle size of the metal nanocrystals 24 is in the range from 1 nanometer to 100 nanometers. The metal nanocrystals can be chosen from the group consisting of gold, silver, copper, tin, nickel, and germanium. Preferably the colloidal solution 20 further contains a stabilizer agent, the stabilizer can be selected for the group consisting of: tetraoctyl ammonium bromide, sodium citrate and poly sodium 4-styrene sulphonate. In this step, the metal nanocrystals 24 are self-assembled on partial or entire exterior surfaces of the carbon nanotubes 14. Preferably, the metal nanocrystals 24 are self-assembled on entire exterior surfaces of the carbon nanotubes 14. The carbon nanotube array 10 is immersed in the colloidal solution for a time period in the range from 5 to 72 hours, preferably 10 to 30 hours.

In step 230, the carbon nanotube array 10 is taken out of the colloidal solution 20. Referring to FIG. 3C, after the previous self-assembly step, the metal nanocrystals 24 are attached to exterior surfaces of the carbon nanotubes 14. The metal nanocrystals 24 are independent from each other as sub-monolayer/monolayer at exterior surfaces of the carbon nanotubes 14 fitted to a Langmuir isotherm.

In step 240, the metal nanocrystals 24 attached to exterior surfaces of the carbon nanotubes 14 is heated to a temperature in air for a period of time to form a continuous polycrystalline metal nanowire array. The temperature is equal to or higher than the melting point of the metal nanocrystals. It should be noted that the metal nanocrystals have a melting point lower than bulk metal. Furthermore, surface melting can occur at a lower temperature due to enhanced mobility of metal atoms. For example, the gold nanocrystals attached to the surface of the carbon nanotubes 14 have a melting point of about 300 degree Celsius. Preferably, the metal nanocrystals 24 are heated for a time period in the range for example from 35 to 60 seconds as in the illustrated embodiment. Referring to FIG. 3D, after the heating step, the neighboring spherical metal nanocrystals are fused together, therefore a polycrystalline metal nanowire array 40 composed of a number of nanowires 30 each having a nanotube core is formed.

It is known that well-aligned carbon nanotube arrays can be obtained by various methods. Therefore, according to the present embodiment, metal nanowire arrays can be synthesized with the nanotube template. The metal nanowire array is suitable for practical applications, such as sensor, catalyst, and thermal interference material.

Finally, it is to be understood that the above-described embodiments are intended to illustrate rather than limit the invention. Variations may be made to the embodiments without departing from the spirit of the invention as claimed. The above-described embodiments illustrate the scope of the invention but do not restrict the scope of the invention. 

1. A method for fabricating a metal nanowire array, comprising the steps of providing a carbon nanotube array comprising a plurality of carbon nanotubes; immersing the carbon nanotube array in a colloidal solution containing a solvent and a plurality of metal nanocrystals dispersed therein, thereby the metal nanocrystals being self-assembled on exterior surfaces of the carbon nanotubes; taking the carbon nanotube array out of the colloidal solution; and heating the metal nanocrystals in a manner such that the metal nanocrystals are fused into the metal nanowire array.
 2. The method as claimed in claim 1, wherein the metal nanocrystals is comprised of a material selected from the group consisting of gold, silver, copper, tin, nickel, and germanium.
 3. The method as claimed in claim 1, wherein the solvent is selected from the group consisting of water, chloroform, hexylene glycol and alcohols containing less than 5 carbon atoms.
 4. The method as claimed in claim 1, wherein the colloidal solution furthermore contains a stabilizer agent.
 5. The method as claimed in claim 4, wherein the stabilizer agent is selected from the group consisting of tetraoctylzrnmonium bromide, sodium citrate, and poly sodium 4-styrensnlfonate.
 6. The method as claimed in claim 1, wherein the carbon nanotube array is immersed in the colloidal solution for a time period in the range from 5 to 72 hours.
 7. The method as claimed in claim 1, wherein the metal nanocrystals are heated to a temperature that is equal to or higher than a melting point of the metal nanocrystals.
 8. The method as claimed in claim 7, wherein the metal nanocrystals are heated for a time period in the range from 35 to 60 seconds.
 9. The method as claimed in claim 7, wherein the carbon nanotubes extend in a common direction.
 10. A metal nanowire array comprising: an array of metal nanowires extending in a common direction, each metal nanowire comprising a core portion comprised of at least one carbon nanotube, and a cladding portion enclosing the core portion therein, the cladding portion being comprised of metal polycrystalline.
 11. A metal nanowire array as claimed in claim 10, wherein the metal polycrystalline is comprised of a material selected from the group consisting of gold, silver, copper, nickel, and germanium. 